Wireless communication systems are widely deployed to provide various types of communication such as voice, data, and so on, for a number of users. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access technique. CDMA systems may provide certain advantages over other types of system such as increased capacity. A CDMA system may be designed to implement IS-95, IS-856, cdma2000, W-CDMA, some other CDMA standard, or any combination thereof. These CDMA standards are well known in the art.
In a wireless communication system, a pilot is often transmitted from a transmission source (e.g., a base station) to a receiver device (e.g., a terminal) to assist the receiver device perform a number of functions. The pilot is typically generated based on a known data pattern (e.g., a sequence of all zeros) and using a known signal processing scheme (e.g., covered with a particular channelization code and spread with a known PN sequence). The pilot may be used at the receiver device for synchronization with the timing and frequency of the transmission source, estimation of the quality of the communication link, coherent demodulation of a data transmission, and possibly other functions such as determination of the specific transmission source having the best link to the receiver device and the highest data rate supportable by the transmission source.
In some CDMA systems (e.g., IS-95, IS-856, and cdma2000 systems), each base station is assigned a specific “offset” of a complex PN sequence used for spreading traffic and pilot data prior to transmission. The use of PN sequences of different offsets by different base stations allows the terminals to distinguish individual base stations based on their assigned PN offsets. Because of variable propagation delays and scattering in the communication link, the signals transmitted from the base stations may reach the terminals at different times. Thus, a terminal typically searches through the received signal at various PN phases (or PN chip offsets) to look for strong instances (or multipaths) of the transmitted signals, which may then be further processed to recover data and other information.
In searching for strong multipaths, the terminal typically performs a number of correlations of the received signal (after preconditioning and digitization) with locally generated PN sequences at various phases. Each correlation results in a high value if the phase of the locally generated PN sequence is aligned with that of the particular multipath being searched, and a low value otherwise.
Because a number of multipaths may be received for a number of base stations assigned with a number of different PN offsets, a PN generator within the terminal needs to generate PN sequences at numerous phases in the search for these multipaths. In the search for strong multipaths from a particular base station, it may be necessary in certain instances to jump the phase of the PN sequence from a current phase to a new phase that may be a large distance away. In certain other instances, it may be necessary to adjust the phase in smaller increments (e.g., in fractions of a PN chip, such as ⅛ chips). The ability to quickly move the PN sequence to the desired phase may improve search performance.
There is therefore a need in the art for techniques to quickly and efficiently generate PN sequences at various arbitrary phases. These PN sequences may be advantageously used to search for strong multipaths from a particular base station in a CDMA communication system.